sphinx/Dockerfile: Install pyarcanist in doc build environment

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Related to T3661

sphinx/Dockerfile

@@ -31,4 +31,4 @@ USER jenkins
 
 RUN --mount=type=cache,id=pip-cache,target=/home/jenkins/.cache/pip,uid=115,gid=120 \
   python3 -m pip install --user --upgrade \
-    pre-commit
+    pre-commit pyarcanist